The present invention relates to a thermal gas sensing apparatus for use in a gas detector.
Gas detectors for sensing the concentration of gases are well known. Such gas detectors may be of various types, including catalytic gas detectors and thermal gas detectors. A catalytic gas detector utilizes a gas sensing circuit having a gas sensing element which detects gas based on combustion of the gas, which heats the gas sensing element. One example of a catalytic gas detector is disclosed in U.S. Pat. No. 4,541,988 to Tozier, et al. In that patent, the gas sensing element is coated with a material that acts as a catalyst to promote combustion of the gas being sensed.
Thermal gas detectors operate based on a different principle. A thermal gas detector utilizes a sensing element, such as a thermistor, through which an electrical current is provided and which generates heat within the sensing element. That heat is dissipated at a rate which depends on the particular gas to which the sensing element is exposed. Since different gases have different heat capacities (i.e. the rate at which the gas can absorb heat from the sensing element), a particular gas can be identified by the rate at which the sensing element is cooled.
Conventional thermal gas detectors are typically provided with a gas sensing circuit composed of a current source that supplies a constant electrical current through a pair of sensing elements, which include a reference element which is isolated from the gas being sensed and a sensing element which is exposed to the gas being sensed. The need to isolate the reference element from the gas increases the complexity of the gas sensing circuit and its cost.